I've thought about something like this. In my mind it all comes down to receipts. We need a system of ubiquitous standardized itemized and cryptographically signed digital receipts. That way everything you buy aggregates into your personal inventory app. Then making a posting is a matter of one click and one photo. The UPC in receipt means a full product listing can be generated automatically. This is crucial since most Craigslist posts don't have sufficient detail about the specific item model and specs. But with a UPC or equivalent that can be pulled in automatically.

Once postings consistently have UPCs attached, you can figure out the market price for everything and push that to people who own the product when the price goes up.

You can also push out product recall and class action lawsuit alerts.

The receipt would really be a proof of purchase, since it's cryptographically signed. So it could be used to make verified third party product reviews without the conflict of interest of the site also selling products.

You could hook your personal inventory database up to your social network to make a lending library with all your friends. Why buy something new when you can search all your friends' stuff and borrow from them?